The invention relates to a process for the preparation of low VOC glycol ether esters.
Coalescing aids are added to waterborne paints (i.e., latex paints) to allow the formation of a continuous polymer or binder film as water evaporates from the system. Without the addition of these coalescing aids, latex polymer spheres are not likely to soften and deform, which is a requirement in film formation. As a result, the polymer cannot act as a binder for the pigments in the paint and no adhesion to the substrate (e.g., interior or exterior wall) can occur. For many years, coalescing aids have been volatile solvents, such as 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, commercially available from Eastman under the trade name TEXANOL.
Volatile organic compound (VOC) emissions contribute to the creation of ozone, a main constituent of smog. In the United States of America, VOC regulations established by the Environmental Protection Agency (EPA) and enforced at the state level dictate the maximum concentration of volatile solvents in paints and other products. In Europe, VOC limits are defined by the 2004/42/EC Solvents Directive for Decorative Paints. Water is a volatile component of waterborne paints but it is exempt from VOC regulations as it does not contribute to smog generation. VOC regulations have become more and more stringent to the point that coalescing aids with zero or very low VOC content are now required in order to meet them.
The VOC content of a paint or substance is currently determined in the United States by EPA Method 24 “Determination of Volatile Matter Content, Water Content, Density, Volume Solids, and Weight Solids of Surface Coatings,” which incorporates several test methods from the American Society for Testing and Materials (ASTM). Volatile matter content is determined using an oven at 110° C. Any substance, other than water, acetone, and a few other exempt compounds, that evaporates after one hour in this oven is considered a VOC. TEXANOL is classified as a 100% VOC by this test. VOC content is expressed as grams VOC/liter of formulation. In the case of a pure, totally volatile substance, its VOC content equals its density at 25° C. Texanol is 100% volatile in the test and has a VOC content of about 948 g/L. For substances that are partially volatile under the test conditions, only the volatile portion is considered VOC.
In the European Union, a substance having a boiling point below 250° C. at 760 mmHg is considered a VOC. New regulations will require normal boiling points above 280° C. for VOC exemption.
US 2012/0258249 and US 2012/0259049 teach the use of various glycol ether esters as zero VOC coalescing aids and clean-up solvents, respectively. Several preparation methods are described in these patent applications. One of these methods is the Fischer esterification reaction, in which a stoichiometric excess of a reactant bearing a hydroxyl group (e.g., an alcohol or glycol ether) and a carboxylic acid are heated in the presence of a catalytic amount of a strong acid (e.g., concentrated sulfuric acid) and an entrainer solvent (i.e., heptane, toluene, etc.) to yield the desired ester. By-product water is removed by azeotropic distillation. An example of this synthesis can be found in “Unitized Experiments in Organic Chemistry” 3rd Edition, by Brewster, VanderWerf, and McEwen, pp. 101-105 (1970). Another method of preparation employs the acid chloride (or dichloride) instead of the carboxylic acid as a reactant. In this case, hydrogen chloride gas is given off instead of water during the reaction. The hydrogen chloride is trapped by the addition of a tertiary amine to the reaction mixture or by means of a water scrubber (“Organic Syntheses, Collective Volume 3,” p. 142 (1955)). Another method of preparation, as disclosed in RD 1987276098 A, involves the transesterification of an alkyl ester of the desired acid with a glycol ether in the presence of a suitable catalyst such as tetraisopropyl titanate. Still another method of esterification uses the acid anhydride as reactant in combination with the azeotropic removal of water in the presence of an entrainer. This latter method is often aimed at producing diesters; see, e.g., CA 2,356,469.
The aforementioned processes often yield reaction mixtures that have undesirable odors and color. Color often arises from decomposition of one of the reactants. Volatile esters can be purified by distillation. However, those of low volatility are difficult to purify sufficiently to obtain a product that is relatively free of odor and color. A cumbersome activated charcoal treatment can be used to improve the color and odor of a relatively non-volatile product. Distillation for the purpose of removing color and odor would be extremely difficult to carry out on an industrial scale for some glycol ether esters or diesters, such as bis-dipropylene glycol n-butyl ether adipate (DPnB adipate), given their high boiling points, which often exceed 450° C. WO 2010/079018 teaches that colored esterification products are produced when the alcohol reactant is not treated beforehand to remove color-inducing impurities.
Additional processes for the preparation of glycol ether esters are described in the literature. EP 0711747 B1 teaches that sulfuric acid and p-toluene sulfonic acid catalysts produce color issues in the synthesis of glycol ether acetates by direct esterification, i.e., the Fischer reaction. Products are recovered and purified by distillation. CA 2,746,599 discloses a direct esterification process using as reactants carboxylic and dicarboxylic acids, C4-C13 alcohols, alkylene glycol monoethers, and polyalkylene glycols monoethers in the presence of a Lewis acid or Bronsted acid catalyst over a broad reaction temperature range (160-270° C.), and requires a minimum alcohol concentration of 30% excess of the stoichiometric amount. That patent teaches that higher temperatures increase the formation of colored by-products.
Aranda et al., in Catal. Lett. (2008) 122:20-25, reported the use of various acids as transesterification catalysts for fatty acids, such as palm oil, for the production of biodiesel. Methanesulfonic and sulfuric acid were the best catalysts, while trichloroacetic acid and phosphoric acid performed poorly.
It would be desirable to have an improved process for the preparation of low-VOC glycol ether esters that would allow production of the desired products in high yield without the need for further treatment, such as charcoal treatment, to remove color and undesirable odor.